It is widely appreciated that people cause serious damage to their teeth and gums by brushing too hard, and there have been a number of designs of toothbrush aimed at overcoming this problem. Several studies have arrived at the conclusion that excessive force during brushing leads to recession on premolars, and also gingival recession, which exposes the underlying cementum, often leading to hypersensitivity, loss of aesthetics, and may be a factor in root caries and root surface abrasion, leading to root fillings.
One solution is a brush, which simply will not transmit excessive force, for example a design disclosed in DE 3724476 where the neck of the toothbrush buckles if the user attempts to brush too hard. This, however, may result in a relatively flimsy product, which may be frustrating to use, as brushing may continually be interrupted.
Another known solution is disclosed in U.S. Pat. No. 5,502,861, which provides a toothbrush with an indicator which signals to the user if excessive force is being applied. This has the advantage that the user is provided with a clear signal that brushing is too hard, and which can be used to learn to brush correctly. It is disclosed in U.S. Pat. No. 5,282,291 that it is thought preferable to make the indicator mechanism an integral part of the brush, rather than an attachment to it; the latter arrangement generally results in a brush which is awkwardly shaped and unnatural to use.
Designs incorporating integral indicators, which are activated by electrical circuits, are known. In the design disclosed in U.S. Pat. No. 5,282,291, components of the circuit are brought into contact as the brush flexes, completing the circuit and triggering the indicator. Often, the user must fit a battery to power the indicator, which due to the limited size of the brush must be small and is, therefore, tricky to fit. In designs where a battery is required, access to the circuitry must be available, for example through a removable cover. This arrangement is inevitably less hygienic: the cover is unlikely to fit exactly flush with the handle surface and debris may collect in any gap between the cover and handle and also in the cavity itself. The device disclosed in DE 3724476 avoids the problem of powering the indicator circuit by relying instead on a piezoelectric component to act as a mechanical-electrical converter producing an electrical signal in response to the applied force. Obviously, these designs require a number of miniature components fitted into a small cavity in the brush handle, which is likely to render them difficult and therefore expensive to manufacture. Also, it is not ideal to locate a battery in a persistently damp environment, especially if it has to be replaced from time to time.
These disadvantages can be overcome by the use of an indicator, which does not require a separate power source or complex circuitry and can be incorporated easily into the body of the brush. U.S. Pat. No. 6,389,636, for example, discloses a toothbrush comprising a piezochromic material, which signals suitable brushing force without the provision of a power supply or electrical circuitry. See also U.S. Pat. No. 6,330,730.
Despite the foregoing developments, there is room for further improvements.